In-situ solution mining of mineral values is a known alternative to surface recovery processes particularly when the later is not economically feasible. Conventionally, in in-situ solution mining processes, the leaching solution is brought into contact with the subterranean deposit through a suitable injection system. The leaching solution or lixiviant may be an alkaline or acidic medium which solubilizes the mineral values as it traverses the ore body. Often, the mineral values in an ore body are subjected to an oxidation step in order to convert them to a soluble form. For example, the tetravalent uranium must be oxidized to its soluble hexavalent form for leaching. The pregnant lixiviant is then withdrawn from the ore body through a suitable production system and treated to recover mineral values therefrom by suitable techniques such as solvent extraction, direct precipitation or by absorption and elution employing an ion exchange resin. All too often, however, and in fact in the majority of cases, the oxidation step will result in the release into the formation of other minerals values than the desired mineral value to be recovered. These other mineral values may not be present in sufficient quantities to justify surface recovery. However they may be present in large enough quantities wherein the formation fluids have to be treated to reduce the concentration of such mineral values or contaminants to environmentally acceptable levels.
For example, recovery of uranium values from subterranean formations involves in the usual methods the oxidation of insoluble tetravalent uranium into soluble uranyl complexes that may be drawn from the formation by leaching. The overall reaction in oxidative in-situ leaching may be described as follows: EQU UO.sub.2 (s)+[O]+3HCO.sub.3.sup.- .fwdarw.UO.sub.2 (CO.sub.3).sub.3.sup.-4 +H.sup.+ +H.sub.2 O
The use of oxygen, however, also solubilizes insoluble molybdate, among other minerals, into the toxic soluble molybdenite ion: EQU MoS.sub.2 +9[O]+3H.sub.2 O.fwdarw.MoO.sub.4.sup.= +6H.sup.+ +2SO.sub.4.sup.=
Current environmental regulations restrict the amount of molybdenum and sulfate permissable in formations after leaching to less than 1 ppm and 600 ppm respectively in New Mexico, for example.
Accordingly, this present invention provides a process for reducing the contaminants level in the formation fluids after the formation has been subjected to oxidative in-situ leaching to recover mineral values therefrom.